Add custom headers for email messages

See http://meta.discourse.org/t/custom-headers-in-emails-site-settings/7480
Prevent errors in case #{rails_root}/tmp does not exist

Dir.mkdir is not recursive. i.e - It doesn't do the equivalent of `mkdir -p` but just plain `mkdir`.
And that is a problem if #{rails_root}/tmp is missing. More here.
http://meta.discourse.org/t/keep-getting-the-same-error-every-start/8214
And add tests for empty string & null
Use FuleUtils.mkdir_p for cleaner code
French translation of user_replied/posted subject

This is a follow-up of the commit 013ad0fdd
Correct caption of 'Make Visible' button

The variable 'visible' is also used for toggling the admin menu visibility,
so it is always true when the visibility toggle is shown. Hence, it's caption
is always 'Make Invisible', even when the topic already is invisible.
Refactoring #1157
Merge pull request #1175 from adrianlang/correctMakeVisibleButtonCaption

Correct caption of 'Make Visible' button
Merge pull request #1173 from nterray/translate_reply_subject

French translation of user_replied/posted subject
Merge pull request #1169 from phanimahesh/patch-1

Prevent errors in case #{rails_root}/tmp does not exist
Merge pull request #1157 from abbat/email-custom-headers

Add custom headers for email messages
Add relative_date_duration site setting